1. Field of the Invention
The present invention relates to a gas mixture intended for delivery, together with a breathing gas, to the lungs of a living being, said gas mixture containing a pre-defined concentration of NO.
2. Description of the Prior Art
All exchanges between blood gases and the atmosphere take place in the lungs, more particularly in the pulmonary alveoli. When the lungs are defective or diseased, the pulmonary alveoli may be afflicted by dysfunction which impairs or prevents the exchange of gas between the blood and the atmosphere. This is also true in the case of defects or disease affecting blood vessels in the lungs. Resistance to the flow of blood through the lungs could then increase, thereby increasing the load on the heart. Very small amounts of nitric oxide (NO) can be supplied to the lungs in order to diagnose pulmonary function and treat the lungs. Nitric oxide relaxes smooth muscle cells in blood vessels and bronchi. Relaxation of the vascular musculature increases the flow of blood through the vessels, thereby improving the exchange of gas between the blood and the atmosphere and reducing the load on the heart. Resistance to flow through the lungs can be determined by measuring the pressure in the pulmonary artery. As a result, the effect of NO administration can be directly measured. If no effect is measurable, the capillaries are either already fully dilated or heavily calcified. Relaxation of smooth muscle cells in the bronchi counteracts bronchospasm and asthma. Nitric oxide gas also diffuses through the lung membrane and can be absorbed by the blood in unlimited quantities, in principle, thereby making it possible to measure the diffusion capacity of the lungs by determining the amount of diffused nitric oxide. NO can also be used in different types of respiratory treatment of adults and children and in the monitoring of respiratory treatment and other diagnostic measures.
In WO 92/10288 the use and effects of NO are described in detail. The document also describes how NO in a mixture of H.sub.2 and O.sub.2 can be delivered to a patient's lung with a ventilator.
European Application 0 570 612 describes a device for delivering very small gas flows to a patient. An accurate concentration of, e.g., NO can be supplied to a patient's lungs with a very small flow of gas.
Since the amounts of nitric oxide supplied are very small, i.e., generally from one or two ppm up to about 100 ppm, measuring the concentration of nitric oxide supplied to a patient's lungs is difficult. The calculations are especially complicated by the circumstance that known NO meters are sensitive to pressure variations, as occur on the inspiratory side of a ventilator system, in principle, there are four types of meters: a chemoluminance meter which records energy quanta emitted in the chemical reaction NO.fwdarw.NO.sub.2 .fwdarw.NO, mass spectrometers, infrared absorption meters and chemical cells which register electron emissions in the reaction NO.fwdarw.NO.sub.2. In order to measure NO concentrations, the known NO meters also need large, continuous flows of breathing gas. Diverting a large flow from the inspiratory flow before it reaches the patient also complicates flow control.
NO is also a highly reactive gas which chemically reacts with oxygen, O.sub.2, to form nitrogen dioxide, NO.sub.2, a toxic gas. As a rule, relatively large amounts of O.sub.2 are supplied in treatment with NO. To prevent NO from reacting with O.sub.2 before it reaches the lungs, NO must be added to the breathing gas (e.g. air and O.sub.2) at a location close to the patient. This also presents major problems in the measurement of the NO concentration because the gas meter must be placed between the lungs and point at which NO is added. This distance must be as short as possible in order to minimize the amount of NO.sub.2 reaching the lungs, but the amount of NO gas supplied must have time to mix with the breathing gas before the NO concentration is measured. In other words, gas meters will be unable to supply a reliable reading of the NO concentration if NO is added to the breathing gas very close to the patient so as to minimize the formation of NO.sub.2. If NO is added to the breathing gas at a greater distance in order to permit correct measurement of the NO concentration, more NO.sub.2 will form. NO.sub.2 molecules can be filtered out in an absorber arranged before the patient, but the concentration of NO could fall below the desired level if numerous NO.sub.2 molecules are formed. Moreover, as noted above, most modern NO meters require a relatively large, continuous flow of gas past the meter. Any diversion of gases from the patient would give NO time to form more NO.sub.2 molecules before the gas reaches the patient's lungs. NO is generally supplied diluted in N.sub.2, primarily to control the small amounts of NO to be supplied.